Presently used chipping heads generating a conical cutting pattern have their knives either attached directly to a rotor of roughly conical shape or have their knives mounted by means of appropriately shaped holders upon the peripheral faces of a polygonal core. One example of a knife holder mounted to a polygonally shaped head may be found in applicant's co-pending U.S. application Ser. No. 08/371,164 filed Jan. 11, 1995.
Major inconveniences are found in existing so-called conical head designs. One is the high cost of fabricating a dynamically balanced rotor of polygonal shape. Another is the uplifting effort from the cutting action being applied entirely to the front mounting screws of the holder, thus generating critical tensional stresses in these elements. The inaccurate lateral positioning of the knife holder, and consequently of the knife itself, due to the diametrical clearances (up to 1/16") between mounting holes and screws in the knife holder, is another major disadvantage. Furthermore, the positioning of the knife on the knife holder is equally inaccurate because similar mounting means are used. Consequently, it is necessary to use in every knife replacement, adjusting screws provided on the knife holder to line up the timber finishing edges of each set of knives on a chip cutting head after the knives have already been jig babbitted and length rectified in the filing room. Knife fabrication of a bent plate design now in common use is expensive as it involves the forming and machining of curved surfaces in addition to cutting the required adjusting slots and babbitting recesses. Equivalent complications exist for the user in the grinding and fitting of such knives for everyday use. Additional inconveniences include the expensive knife holder construction from a steel casting, the extra weight involved and, at times, the availability problems of the castings. There is also the problem of abrasion wear on the majority of knife holders which are of a four-wall construction, plus some chip deterioration due to impact with the holder outer wall. Desirable diameter reduction of the cutting circle for heads using the four-sided knife holder is limited due to the space requirements between the holders to allow chip exit out of the cutting area. Finally, erosion of the counterknife working face and leading edge sometimes allows the introduction of wood fiber under the tip of the counterknife; the progressive buildup of such fiber causes stretching and sometimes breaking of the knife retaining screws. Any loosening of the knife retaining screws which thread into the counterknife will also allow its leading edge to admit sufficient wood fiber to generate destructive pressures between the knife and knife holder faces.